1. Field of the Invention
This invention relates to silicone compositions which are made electrically conductive through the addition of metal particles.
2. Background Information
The electronic industry uses electrically conductive adhesives to attache silicon chips to a substrate. Metal alloys find use in this application, but they require very high temperatures during processing to achieve flow out and bonding, are expensive, and have a very high modulus. Epoxy resins filled with silver particles have found use in this application. The filled epoxies ar easier to process and for along time provided the microelectronic industry with acceptable performance. Silver filled epoxies are, however, rigid materials having a high modulus. With the current trend toward larger devices there is a need for the electrically conductive adhesive to provide stress-relief. Otherwise, during thermal cycling, the device and/or the substrate can crack and fail due to induced stresses. The high modulus epoxies do not provide this stress relief. Additionally, the microelectronic industry has beed for high reliability devices. Current silver filled epoxies typically do not possess the purity to satisfy these needs. In addition, a suitable composition must adhere to the components, have a volume resistivity of less than 10.sup.-2 ohm-cm, remain stable over a temperature range, and maintain electrical conductivity during aging at elevated temperatures.
U.S. Pat. No. 3,140,342, issued July 7, 1964, teaches that effective seals for electrical closures can be formed from a compressible or resilient plastic loaded or filled with metal particles. When the metal particles are a noble metal of an average particle size in the range of 0.1 to 20 micrometers, and present in the range of 20 to 66 volume percent, the gasket formed can have an electrical resistance of less than 10.sup.-3 ohm-cm.
U.S. Pat. No. 3,576,387, issued Apr. 27, 1971, related to heat shrinkable electromagnetic shields teaches the addition of silver plated copper powder having an average particle size of from 2 to 3 micrometers into a silicone, pressure-sensitive adhesive.
A discussion of the use of silver particles in epoxy adhesives in the article, "Development of Electrical Conduction in Silver-filled Epoxy Adhesives", A. J. Lovinger; J. Adhesion, 1979, Vol. 10, pp 1-15, Gordon and Breach Science Publishers Ltd., shows that there is no conduction using silver flakes unless the adhesive is cured at elevated temperature.
An electrically conductive stock useful for electromagnetic shielding is disclosed in U.S. Pat. No. 4,011,360, issued Mar. 8, 1977. The stock comprises a mixture of siloxane polymer, an aminosilane or aminosilazane curing agent and electrically conductive particles having an outer surface of noble metal. This patent teaches that other curing systems may not be successful.
Adhesives are taught in U.S. Pat. No. 4,051,454, issued Sept. 27, 1977, based upon vinyl containing organopolysiloxanes, organopolysiloxanes having Si-bonded hydrogen atoms and catalysts which promote the addition of the Si-bonded hydrogen atoms to the vinyl groups. The adhesives may be combined with conductive carbon black, graphite and powdered metals, such as copper, aluminum and-or silver to give electrical conductivity, in amounts generally of form 0.05 to 3 percent by weight.
U.S. Pat. No. 4,250,075, issued Feb. 10, 1981 teaches compositions which cure to electrically conductive silicone elastomers. The composition is required to contain greater than 15 percent by weight of carbonaceous particles and greater than 2 percent by weight of phenyl radicals. The useful life of an ignition cable made with the composition is stated to be dependent upon the amount of phenyl radical present.
U.S. Pat. No. 4,695,404, issued Sept. 22, 1987 teaches a polymeric composition having an electrical resistivity in the used state of no more than 10 .sup.-4 ohm-cm. The composition comprises a polymer chosen from an epoxy resin, silicone, fluorosilicone, polyurethane, or long chain hydrocarbon filled with from 70 to 90 percent by weight of silver particles in the form of flakes having a volume electrical resistivity in a compressed free flow state of not more than 2.times.10.sup.-4 ohm-cm. The composition has a stable volume electrical resistivity of 0.00001 ohm-cm or less.
There is not teaching in this art as to how to select the silicone component of the composition so as to obtain the maximum electrical conductivity in a metal filled composition.
There is no teaching in the art as to how to choose a silicone composition so as to obtain a high electrical conductivity which is retained upon aging at elevated temperatures.